It is well known to utilize, for instance, Molecular Beam Epitaxy (MBE) systems to deposit epitaxial layers upon process elements such as semiconductor substrates. It is also well known to utilize Reflection High Energy Electron Diffraction (RHEED) and Low Energy Electron Diffraction (LEED) systems, mounted to (MBE) systems at manufacture, as a source of "real-time" signals for use in monitoring and possibly controlling (MBE) epitaxial layer deposition rates.
Briefly, a (MBE) system is comprised of a vacuum chamber in which one or more effusion cells are present, and into which vacuum chamber can be placed a process element. During use thereof said effusion cells serve as the source of elements or compounds etc. which are to be deposited as an epitaxial layer upon said process element. When "real-time" monitor and/or control of the thickness or rate of an epitaxial layer deposition upon a process element in an (MBE) system is required a (RHEED) or (LEED) system is commonly mounted to said (MBE) system such that a beam of electrons is caused thereby to impinge upon said substrate at a glancing, (eg. typically at an eight-eight degree angle to the normal to said process element for (RHEED) systems and a considerably lesser angle for (LEED) systems), incident angle and be reflected into a detection System wherein a signal is produced which can be used to monitor deposition rate, possibly control effusion cell operation and/or terminate the deposition etc. It is emphasised that (MBE) systems are commonly fitted with electron diffraction (RHEED) systems at manufacture.
It is to be appreciated that "real-time" signals for use in monitoring and in the control of epitaxial deposition in (MBE) systems can also be derived from Ellipsometer (E) and Polarimeter (P) systems, and that signals from such systems can provide information regarding not only deposited epitaxial layer growth rate and thickness, but also composition, roughness and even the temperature thereof during deposition. It is emphasised that Ellipsometer derived signals can be easily utilized in process control with respect to a number of process element parameters. Signals from (RHEED) systems, however, typically provide monitoring information regarding epitaxial layer deposition rate and thickness only within a limited range and are rarely used in a control mode. While benefits of utilizing (E) and (P) systems rather than (RHEED) systems are thus identified, a problem exists in interfacing (E), (P) and (MBE) systems because major and costly, (in terms of time and money), modifications must typically be made to a standard (MBE) system to allow application of (E), (P) or functionally similar systems thereto so that a beam of light produced thereby can be caused to impinge upon a process element therein at an optimum angle of incidence, near the Brewster angle. (Those skilled in the art of ellipsometry will understand the reference to the Brewster angle to identify that angle of incidence at which the Rp component of a polarized beam of light reflected from a process element, as monitored by intensity measurements, becomes a minimum and in some cases essentially zero (0) while the Rs component remains a relatively large percentage, (eg. 35%+), as compared to respective incident beam intensities). Accepted teachings and understanding of those with ordinary skill in the art of (E) applications are that any deviation from a Brewster angle of incidence, (especially an angle greater than the Brewster angle as measured with respect to a normal to a process element), lead to reduced (E) process element parameter change (eg. epitaxial deposition), detection sensitivity. That is, no known teachings remotely suggest that an (E) system should be applied to monitoring, for instance, epitaxial deposition upon a process element in "real-time", wherein the angle at which a produced beam of light is caused to be incident upon a process element significantly exceeds the Brewster angle. A book by Azzam and Bashara titled "Ellipsometry and Polarized Light", published by North Holland, 1989 makes this very clear, as does a paper by Snyder et al. titled "Variable angle of Incidence Spectroscopic Ellipsometer: Application to GaAs-AlxGal-x As Multiple Hetrostructures", J. App. Physics, Vol. 60(9), 1986. The summary of said paper states " . . . the sensitivity of Delta was found to be strongly peaked near the principal angle, . . . ". (Note that the "Principal angle" is, for the purposes of this Disclosure, to be interpreted as being simply alternate terminology for "Brewster angle").
In addition, inspection of a patent to Hartley, U.S. Pat. No. 4,770,895, shows very well that conventional understanding in the relevant art provides that a Spectroscopic Ellipsometer (SE) system should be oriented so that a polarized light beam produced thereby is incident at a less glancing angle to a process element surface during use, than is a beam of electrons from a (RHEED) system.
In summary, when a single wavelength polarized beam of light is-incident upon a process element, sensitivity to epitaxial layer thickness and other parameters achievable by an (E), (P) or functionally similar system has typically been observed to decrease when the angle of incidence thereof upon a process element exceeds the Brewster angle.
Continuing, recent developments in (SE) technology have allowed the Inventors of the present invention to apply and detect a multiplicity of polarized light beam wavelengths simultaneously. This capability allowed the Inventors of the present invention to discover the extremely surprising result that when a multiwavelength (SE) polarized beam of light is impinged upon a process element at an incident angle of from approximately seventy-eight (78) to eighty-five (85) degrees to a normal to said process element, (which later incident angle is comparable to that at which electron beams are incident upon process elements by (RHEED) systems, and which incident angle is well beyond the Brewster angle associated with (SE) systems for typical substrates), signals derivable from certain of the wavelengths present provide acceptably good sensitivity to changes in process element parameters of interest such as process element thickness, composition, surface roughness and temperature. That is, even when the angle of incidence of a (SE) polarized beam of light is beyond the Brewster angle, (which for the case of semiconductor process elements is approximately seventy-five (75) degrees), some wavelengths in a multiwavelength beam of light can be utilized in development of strong signals appropriate for use in real-time monitoring and control of (MBE) epitaxial deposition because they demonstrate adequate high sensitivity to changes in said process element parameters, control of which is desired. It is noted that as epitaxial layers are grown or material composition changes, the wavelengths giving optimum signals in (SE) and functionally similar systems may change. However, there will generally be one or more wavelengths, perhaps closely bunched or perhaps distinctly separated, which demonstrate good sensitivity.
Because of the superior process element parameter detection capabilities of (E) systems as compared to (RHEED) systems, which superior detection capabilities were alluded to infra, it should be clear that utility would be provided by use of (E) systems in development of signals appropriate for use in the control of epitaxial layer deposition in (MBE) systems. Until recently however, application of (E) systems to typical (MBE) systems was thought to require an (E) system oriented with respect to a (MBE) system such that a polarized beam of light produced thereby was caused to be incident upon a process element therein at or very near the Brewster angle with respect thereto. Retro-fit of (SE) systems to existing (MBE) systems, again as alluded to infra, is as a result, typically expensive and time consuming, and risks contamination of the (MBE) chamber with impurities.
The present invention is found in the extremely surprizing discovery that an (E) system can be retro-fitted to existing (MBE) systems utilizing existing (RHEED) or (LEED) interface systems and successfully utilized when so oriented to produce signals which can be utilized in the monitor and control of epitaxial deposition onto process elements in said (MBE) system. This, of course, forces use of such a retro-fitted (E) system in a mode wherein the angle of incidence of a polarized beam of light produced thereby, upon said process element, greatly exceeds the Brewster angle where the process element is a semiconductor. The present invention, however, teaches that proper selection of one or more wavelengths from a multiplicity of wavelengths present in a beam of light, (preferably polarized), so incident, allows use of a (SE) system in development of signals appropriate for use in monitor and control of (MBE) system operation. In view of conventional knowledge and practice in the area of (SE) systems and applications thereof, this is a very surprising discovery. The present invention then makes possible an approach to simple economical retro-fit application of (E) and functionally similar systems to existing process element processing systems (eg. (MBE) systems), which are, at manufacture, fitted with, for instance, (RHEED) or (LEED) systems. The present invention provides an unexpected, easily implemented, solution to a long felt need. Said need being best demonstrated as a simple cost effective application of (E) systems, preferably (SE) systems, to (MBE) systems which are not designed for use therewith at manufacture.
The present invention is not to be considered as limited to retro-fit application in (MBE) systems which are initially manufactured with (RHEED) interface systems present. Even in the case of new (MBE) systems combining (RHEED) and (SE) ports simplifies the construction and preserves valuable space in the (MBE) chamber which otherwise has to be used to mount the (SE) to allow use at the Brewster angle. As well, the method of the present invention can be practiced even in the absence of an (MBE) system.
It is also to be understood that (RHEED) and (LEED) systems require a vacuum through which a produced beam of electrons is caused to flow toward a process element. Light Beams do not require a vacuum, hence, a (MBE) system utilizing a (E) or functionally similar system can have the vacuum present therewithin selected based singularly upon effusion cell operation requirements.
The present invention is suitable for use with process elements such as semiconductors, crystals of superconductors, optoelectronics, ferroelectric materials etc.
Finally, the reader is referred to the cited book by Azzam and Bashara, (incorporated by reference herein), and other standard references for insight to ellipsometry, spectroscopic ellipsometry, polarimetry, polarized light reflectance and other light beam producing source based techniques. It is directly stated however that all techniques involve the impinging of a light beam comprised of one or more wavelengths upon a process element, and monitoring changes in the light beam effected by interaction therewith. Ellipsometry utilizes polarized light and effected changes in orthoganally related Rp and Rs components, (and ratios thereof), are investigated. Polarimetry is a very much similar technique but in which rotational direction of the light beam polarization is also important. A technique termed Polarized Light Reflectance utilizes only one of the Rp and Rs components of a polarized light beam. Spectroscopic Ellipsometry involves use of a multiplicity of wavelengths simultaneously. Any light beam based technique which utilizes single or multiple wavelength polarized or nonpolarized light is to be considered within the scope of the present invention.